1. Field of the Invention
The present invention relates to a control circuit for controlling the charging current at the end of the charge phase for batteries, especially for lithium batteries. The charging current at the end of the charging phase is called the charge end current.
2. Discussion of the Related Art
In the field of power management for mobile phones, there are circuits adapted for monitoring the main battery charge level and backup, during the standard working of the device and/or during the charge phase of the batteries. These circuits also have the essential task of managing the current consumption during the different operative modes of the device.
Usually the charge operation of a battery is performed by a constant value current source and is carried out according to the type of battery used in the particular device.
For example, in nickel-cadmium type (NiCd) or nickel-metal-hydrid type (NiMH) batteries, the technique used to end the charge phase consists usually in the monitoring of the time change of the charge voltage and/or of the battery temperature. In the case of lithium type (Li) batteries the battery charger used is a current limited voltage source.
The end phase of charging a lithium battery is defined by the simultaneous control of two parameters, the charge voltage level and the current flowing in the battery.
In order to charge the battery at its maximum capacity, it is necessary that the charge end current supplied by the charge circuit is always less than that supplied by the battery.
The input/output integrated circuits between the battery charger and the battery presently use a comparator that turns off the charge switch when the battery voltage is near the battery charger voltage and the input/output circuits include an output stage that can change its output resistance so as to reduce the current that flows from the battery charger to the battery.
This logic turns off at a charge end current greater than that which would provide for optimum charging. The lithium battery, therefore, is not charged to its maximum capacity.
Moreover, makers of mobile phones want to have integrated devices with lower output resistance to reduce power consumption and therefore to improve the power management of the available charge in the battery.
In view of the state of the art described, it is an object of the present invention to modulate the output resistance of the battery-charger and battery input/output so that it is possible to have a charge end current near zero.
According to the present invention, this and other objects are achieved by a control circuit for controlling the charge end current of batteries, especially for lithium batteries, comprising an input/output circuit, placed between a battery charger and a battery, comprising a transconductance operational amplifier and a control circuit, and an output stage, comprising two transistors, one of which is controlled by said control circuit, wherein the other transistor is controlled by said transconductance operational amplifier so as to increase the value of the total resistance of said output stage as a function of the voltage difference between the positive pole of said battery charger and the positive pole of said battery so to provide a smaller turning off current of said output stage.
As a result of the present invention it is possible to make a device by means of which the current that can flow in the battery during the end of the charging phase does not have a minimum lower threshold but it can tend to or be made zero.
Moreover such circuit solution is flexible and it is applicable also to future generations of lithium batteries because the charge end current will be smaller and smaller and, finally, it does not involve threshold choices and therefore it is a robust solution from the point of view of the technology.